The evolution of the immune system resulted in vertebrates in a highly effective network based on two types of defense: the innate and the adoptive immunity.
In contrast to the evolutionary ancient innate immune system that relies on invariant receptors recognizing common molecular patterns associated with pathogens, the adoptive immunity is based on highly specific antigen receptors on B cells (B lymphocytes) and T cells (T lymphocytes) and clonal selection.
While B cells raise humoral immune responses by secretion of antibodies, T cells mediate cellular immune responses leading to destruction of recognized cells.
T cells play a central role in cell-mediated immunity in humans and animals. The recognition and binding of a particular antigen is mediated by the T cell receptors (TCRs) expressed on the surface of T cells.
The T cell receptor (TCR) of a T cell is able to interact with immunogenic peptides (epitopes) bound to major histocompatibility complex (MHC) molecules and presented on the surface of target cells. Specific binding of the TCR triggers a signal cascade inside the T cell leading to proliferation and differentiation into a maturated effector T cell. To be able to target a vast variety of antigens, the T cell receptors need to have a great diversity.
This diversity is obtained by genetic rearrangement of different discontinuous segments of genes which code for the different structural regions of TCRs. TCRs are composed of one α-chain and one β-chain or of one γ-chain and one δ-chain. The TCR α/β chains are composed of an N-terminal highly polymorphic variable region involved in antigen recognition and an invariant constant region. On the genetic level, these chains are separated into several regions, a variable (V) region, a diversity (D) region (only β- and δ-chain), a joining (J) region and a constant (C) region. The human β-chain genes contain over 60 variable (V), 2 diversity (D), over 10 joining (J) segments, and 2 constant region segments (C). The human α-chain genes contain over 50 V segments, and over 60 J segments but no D segments, as well as one C segment. The murine β-chain genes contain over 30 variable (V), 2 diversity (D), over 10 joining (J) segments, and 2 constant region segments (C). The murine α-chain genes contain almost 100 V segments, 60 J segments, no D segments, but one C segment. During the differentiation of T cells, specific T cell receptor genes are created by rearranging one V, one D (only β- and δ-chain), one J and one C region gene. The diversity of the TCRs is further amplified by imprecise V-(D)-J rearrangement wherein random nucleotides are introduced and/or deleted at the recombination sites. Since the rearrangement of the TCR gene loci occurs in the genome during maturation of T cells, each mature T cell only expresses one specific α/β TCR or γ/δ TCR.
MHC and antigen binding is mediated by the complementary determining regions 1, 2 and 3 (CDR1, CDR2, CDR3) of the TCR. The CDR3 of the β-chain which is most critical for antigen recognition and binding is encoded by the V-D-J junction of the rearranged TCR β-chain gene.
The TCR is a part of a complex signaling machinery, which includes the heterodimeric complex of the TCR α- and β-chains, the co-receptor CD4 or CD8 and the CD3 signal transduction module (FIG. 1). While the CD3 chains transfer the activation signal inside the cell, the TCR α/β heterodimer is solely responsible for antigen recognition. Thus, the transfer of the TCR α/β chains offers the opportunity to redirect T cells towards any antigen of interest.
Immunotherapy
Antigen-specific immunotherapy aims to enhance or induce specific immune responses in patients to control infectious or malignant diseases. The identification of a growing number of pathogen- and tumor-associated antigens (TAA) led to a broad collection of suitable targets for immunotherapy. Cells presenting immunogenic peptides (epitopes) derived from these antigens can be specifically targeted by either active or passive immunization strategies.
Active immunization tends to induce and expand antigen-specific T cells in the patient, which are able to specifically recognize and kill diseased cells. In contrast passive immunization relies on the adoptive transfer of T cells, which were expanded and optional genetically engineered in vitro (adoptive T cell therapy).
Vaccination
Tumor vaccines aim to induce endogenous tumor-specific immune responses by active immunization. Different antigen formats can be used for tumor vaccination including whole cancer cells, proteins, peptides or immunizing vectors such as RNA, DNA or viral vectors that can be applied either directly in vivo or in vitro by pulsing of DCs following transfer into the patient.
The number of clinical studies where therapy-induced immune responses can be identified is steadily increasing due to improvements of immunization strategies and methods for detection of antigen-specific immune responses (Connerotte, T. et al. (2008). Cancer Res. 68, 3931-3940; Schmitt, M. et al. (2008) Blood 111, 1357-1365; Speiser, D. E. et al. (2008) Proc. Natl. Acad. Sci. U.S.A 105, 3849-3854; Adams, S. et al. (2008) J. Immunol. 181, 776-784). However, in most cases detected immune responses cannot systemically be correlated with clinical outcomes (Curigliano, G. et al. (2006) Ann. Oncol. 17, 750-762; Rosenberg, S. A. et al. (2004) Nat. Med. 10, 909-915).
The exact definition of peptide epitopes derived from tumor antigens may therefore contribute to improve specificity and efficiency of vaccination strategies as well as methods for immunomonitoring.
Adoptive Cell Transfer (ACT)
ACT based immunotherapy can be broadly defined as a form of passive immunization with previously sensitized T cells that are transferred to non-immune recipients or to the autologous host after ex vivo expansion from low precursor frequencies to clinically relevant cell numbers. Cell types that have been used for ACT experiments are lymphokine-activated killer (LAK) cells (Mule, J. J. et al. (1984) Science 225, 1487-1489; Rosenberg, S. A. et al. (1985) N. Engl. J. Med. 313, 1485-1492), tumor-infiltrating lymphocytes (TILs) (Rosenberg, S. A. et al. (1994) J. Natl. Cancer Inst. 86, 1159-1166), donor lymphocytes after hematopoietic stem cell transplantation (HSCT) as well as tumor-specific T cell lines or clones (Dudley, M. E. et al. (2001) J. Immunother. 24, 363-373; Yee, C. et al. (2002) Proc. Natl. Acad. Sci. U.S. A 99, 16168-16173).
Adoptive T cell transfer was shown to have therapeutic activity against human viral infections such as CMV. While CMV infection and reactivation of endogenous latent viruses is controlled by the immune system in healthy individuals, it results in significant morbidity and mortality in immune compromised individuals such as transplant recipients or AIDS patients.
Riddell and co-workers demonstrated the reconstitution of viral immunity by adoptive T cell therapy in immune suppressed patients after transfer of CD8+ CMV-specific T cell clones derived from HLA-matched CMV-seropositive transplant donors (Riddell, S. R. (1992) Science 257, 238-241).
As an alternative approach polyclonal donor-derived CMV- or EBV-specific T cell populations were transferred to transplant recipients resulting in increased persistence of transferred T cells (Rooney, C. M. et al. (1998) Blood 92, 1549-1555; Peggs, K. S. et al. (2003) Lancet 362, 1375-1377).
For adoptive immunotherapy of melanoma Rosenberg and co-workers established an ACT approach relying on the infusion of in vitro expanded autologous tumor-infiltrating lymphocytes (TILs) isolated from excised tumors in combination with a non-myeloablative lymphodepleting chemotherapy and high-dose IL2. A recently published clinical study resulted in an objective response rate of ˜50% of treated patients suffering from metastatic melanoma (Dudley, M. E. et al. (2005) J. Clin. Oncol. 23: 2346-2357).
However, patients must fulfill several premises to be eligible for ACT immunotherapy. They must have resectable tumors. The tumors must generate viable TILs under cell culture conditions. The TILs must be reactive against tumor antigens, and must expand in vitro to sufficient numbers. Especially in other cancers than melanoma, it is difficult to obtain such tumor-reactive TILs. Furthermore, repeated in vitro stimulation and clonal expansion of normal human T lymphocytes results in progressive decrease in telomerase activity and shortening of telomeres resulting in replicative senescence and decreased potential for persistence of transferred T cells (Shen, X. et al. (2007) J. Immunother. 30: 123-129).
An approach overcoming the limitations of ACT is the adoptive transfer of autologous T cells reprogrammed to express a tumor-reactive TCR of defined specificity during short-time ex vivo culture followed by reinfusion into the patient. This strategy makes ACT applicable to a variety of common malignancies even if tumor-reactive T cells are absent in the patient. Since the antigenic specificity of T cells is rested entirely on the heterodimeric complex of the TCR α- and β-chain, the transfer of cloned TCR genes into T cells offers the potential to redirect them towards any antigen of interest. Therefore, TCR gene therapy provides an attractive strategy to develop antigen-specific immunotherapy with autologous lymphocytes as treatment option. Major advantages of TCR gene transfer are the creation of therapeutic quantities of antigen-specific T cells within a few days and the possibility to introduce specificities that are not present in the endogenous TCR repertoire of the patient.
Several groups demonstrated, that TCR gene transfer is an attractive strategy to redirect antigen-specificity of primary T cells (Morgan, R. A. et al. (2003) J. Immunol. 171, 3287-3295; Cooper, L. J. et al. (2000) J. Virol. 74, 8207-8212; Fujio, K. et al. (2000) J. Immunol. 165, 528-532; Kessels, H. W. et al. (2001) Nat. Immunol. 2, 957-961; Dembic, Z. et al. (1986) Nature 320, 232-238).
Feasibility of TCR gene therapy in humans was recently demonstrated in clinical trials for the treatment of malignant melanoma by Rosenberg and his group. The adoptive transfer of autologous lymphocytes retrovirally transduced with melanoma/melanocyte antigen-specific TCRs resulted in cancer regression in up to 30% of treated melanoma patients (Morgan, R. A. et al. (2006) Science 314, 126-129; Johnson, L. A. et al. (2009) Blood 114, 535-546).
Target Structures for Antigen-Specific Immunotherapy
The discovery of multiple tumor-associated antigens (TAAs) has provided the basis for antigen-specific immunotherapy concepts (Novellino, L. et al. (2005) Cancer Immunol. Immunother. 54, 187-207). TAAs are unusual proteins expressed on tumor cells due to their genetic instability, which have no or limited expression in normal cells. These TAAs can lead to specific recognition of malignant cells by the immune system.
Molecular cloning of TAAs by screening of tumor-derived cDNA expression libraries using autologous tumor-specific T cells (van der Bruggen, P. et al. (1991) Science 254, 1643-1647) or circulating antibodies (Sahin, U. et al. (1995) Proc. Natl. Acad. Sci. U.S.A 92, 11810-11813), reverse immunology approaches, biochemical methods (Hunt, D. F. et al. (1992) Science 256, 1817-1820), gene expression analyses or in silico cloning strategies (Helftenbein, G. et al. (2008) Gene 414, 76-84) led to a significant number of target candidates for immunotherapeutic strategies. TAAs fall in several categories, including differentiation antigens, overexpressed antigens, tumor-specific splice variants, mutated gene products, viral antigens and the so-called cancer testis antigens (CTAs). The cancer testis family is a very promising category of TAAs as their expression is restricted to the testis and a multitude of different tumor entities (Scanlan, M. J. et al. (2002) Immunol. Rev. 188, 22-32). Until now more than 50 CT genes have been described (Scanlan, M. J. et al. (2004) Cancer Immun. 4, 1) and some of them have been addressed in clinical studies (Adams, S. et al. (2008) J. Immunol. 181, 776-784; Atanackovic, D. et al. (2004) J. Immunol. 172, 3289-3296; Chen, Q. et al. (2004) Proc. Natl. Acad. Sci. U.S.A 101, 9363-9368; Connerotte, T. et al. (2008). Cancer Res. 68, 3931-3940; Davis, I. D. et al. (2004) Proc. Natl. Acad. Sci. U.S.A 101, 10697-10702; Jager, E. (2000) Proc. Natl. Acad. Sci. U.S.A 97, 12198-12203; Marchand, M. et al. (1999) Int. J. Cancer 80, 219-230; Schuler-Thurner, B. et al. (2000) J. Immunol. 165, 3492-3496).
In spite of the growing number of attractive target structures for immunotherapeutic approaches specific T cell clones or lines of defined HLA restriction do only exist for a few of them (Chaux, P. et al. (1999) J. Immunol. 163, 2928-2936; Zhang, Y. et al. (2002) Tissue Antigens 60, 365-371; Zhao, Y. et al. (2005) J. Immunol. 174, 4415-4423). For the majority of CTAs, including TPTE, even evidence for specific T cell responses is missing.